1. Field of the Invention
This invention relates to an epitaxial structure for GaP light-emitting diodes.
2. Description of the Related Art
An epitaxial structure for a light-emitting diode is normally obtained by epitaxially growing a plurality of semiconductor layers on a semiconductor substrate to form a p-n junction. In an epitaxial structure for a green light-emitting diode using gallium phosphide (GaP), an active layer is formed by growing n-conductivity type (hereinafter "n-type") and p-conductivity type (hereinafter "p-type"); GaP layers on an n-type GaP single crystal substrate, and to obtain a high electro-luminescence efficiency, nitrogen is normally used in doping an n-type GaP layer. While the electro-luminescence efficiency of GaP green light-emitting diodes has been improved to some extent by use of a nitrogen dopant, there has been a need for even higher electro-luminescence efficiency levels for applications such as large displays for outdoor use, for example. Not only for epitaxial structures for GaP light-emitting diodes, but for epitaxial structures for light-emitting diodes in general, improving the crystallinity is of key importance for realizing high electro-luminescence efficiency. However, GaP lightemitting diodes, in particular, have an indirect type emission mechanism that results in a low emission efficiency and makes these light-emitting diodes susceptible to the influence of the crystallinity.
A widely used index of crystallinity is the etch pit density (EPD) of the crystal surface. This is an evaluation method that utilizes the pits formed just at crystal defect portions by a specific etching fluid. With respect to GaP, the EPD is obtained as a count of the number of pits per square centimeter of surface formed by an etching method known as Richard & Crocker etching ("RC etching").
FIG. 3 shows the relationship between substrate EPD and the emission efficiency of the LED formed on the substrate. From FIG. 3 it can be seen that the lower the EPD of the substrate, the higher tends to be the emission efficiency of the LED. As such, developing an epitaxial structure for a high electroluminescence efficiency GaP light-emitting diode comes down to developing a substrate having a low EPD.
However, while there are various methods of fabricating a GaP single crystal substrate, in general it is difficult to reduce the EPD. Instead, JP-B-HEI-2-18319, for example, discloses a method for reducing the effect of the EPD. In accordance with the method, in the epitaxial growth system the GaP substrate is contacted with the melt first to form an n-type GaP layer (hereinafter referred to as the "n.sub.0 layer") on the GaP substrate by a supercooled growth method. The substrate is then separated from the melt and the temperature of the epitaxial growth system is again elevated, the substrate on which the n.sub.0 layer has been grown is contacted with a fresh melt as a starting material and the temperature is further elevated, the n.sub.0 layer reverts back to a melt, following which a normal process is used to fabricate a p-n junction by forming another n-layer, (hereinafter referred to as the "n.sub.1 layer"), an n-layer in which nitrogen is doped (hereinafter referred to as the "n.sub.2 layer") and a p-layer in which zinc is used as a dopant, in a continuous epitaxial growth operation. In this method, the no layer grown in the first half of the process does not contribute directly to light emission. The disclosure teaches that all or part of the n.sub.0 layer is melted back to form a fresh melt in the second half of the process. Also, the n.sub.0 layer melt and heating program are separated from the epitaxial process used in the latter half to form the p-n junction, and as such, layer thickness and other growth conditions can be set independently. Since the n.sub.0 layer is thus an epitaxial layer that comes between the GaP substrate and the latter half epitaxial process, hereinbelow this n.sub.0 layer is referred to as a buffer layer.
Inasmuch as the n.sub.1 layer grown at the beginning of the latter half of the epitaxial process also does not contribute to the light emission, the n.sub.1 layer also corresponds to a buffer layer. However, as in the latter half process, in which the n.sub.1 layer is included, growth is by the supercooled growth method, the sum thickness of the epitaxial layers is constant. Thus, unlike the case of the n.sub.0 layer, changing the thickness of the n.sub.1 layer results in a related change in the thickness of the n.sub.2 layer and of the p-layer. Also, the temperature at which growth of the n.sub.1 layer is completed becomes the growth starting temperature of the n.sub.2 layer, i.e., the active layer. Thus, since n.sub.1 layer growth conditions directly affect the n.sub.2 layer growth conditions, strictly speaking the n.sub.1 layer is differentiated from then n.sub.0 layer which is a buffer layer, and constitutes part of the active layer. According to the disclosure, using a method that provides this buffer layer results in an improvement of 0.4% or more in the emission efficiency of the active layer. However, based only on that, it is difficult to realize a GaP light-emitting diode having a high enough electro-luminescence efficiency for outdoor use.
An object of the present invention is to provide an epitaxial structure for a high electroluminescence efficiency GaP light-emitting diode having an improved buffer layer and a decreased EPD on the surface on which the active layer is formed.